Conventionally, a filtration material has been used in order to collect dust in the air. Since it is preferable that such a filtration material has a low pressure drop and a high dust filtering efficiency, a charged filtration material is used to satisfy such requirements. As the charged filtration material, a filtration material prepared by charging two or more types of fibers different in constituent resin by friction, based on the difference in the triboelectric series, is known.
For example, the applicant proposed “a charged nonwoven fabric consisting of a plurality of clean fiber components, wherein the fiber components are charged by friction therebetween, characterized in that the plural fiber components comprise polyolefin-based fibers, and acrylic-based fibers spun by an inorganic solvent” (Patent literature 1). This charged nonwoven fabric had the above-mentioned properties, i.e., a low pressure drop and a high dust filtering efficiency. However, when a filtration material is used, such as a case where pleat processing is carried out, and its periphery is fixed with an outer frame to prepare a filter element, there is a case where a certain degree of rigidity is needed. However, the rigidity of the charged nonwoven fabric was insufficient, and it was unlikely to be used as a filtration material.
Further, in addition to Patent literature 1, the applicant proposed “a charged-type air filter, wherein a prefilter and a main filter are laminated, and at least one of the filters is charged, characterized in that the prefilter is a triboelectrically-charged-type nonwoven fabric comprising clean polyolefin-based fibers, and clean acrylic-based fibers spun by an inorganic solvent, and the main filter is a melt-blown nonwoven fabric” (Patent literature 2), and “a charged filter in which a prefilter layer, a main filter layer, and a backup filter layer are laminated, characterized in that the prefilter layer is made of a charged nonwoven fabric having a mass per unit area of 40-120 g/m2, in which fibers having an average fineness of 1-6 dtex are entangled with each other, the main filter layer is made of a charged nonwoven fabric consisting of fibers having an average fiber diameter of 10 μm or less, the backup filter layer is made of a charged nonwoven fabric having a mass per unit area of 100-300 g/m2, in which fibers having an average fineness of 1-6 dtex are entangled with each other, and the mass per unit area of the backup filter layer is higher than that of the prefilter layer” (Patent literature 3).
These charged filters disclose that, in addition to the above-mentioned triboelectrically-charged-type nonwoven fabric, a melt-blown nonwoven fabric, another triboelectrically-charged nonwoven fabric, or the like may be laminated, and disclose methods of unifying a laminate, such as ultrasonic seal, an adhesive, fiber bonding, or the like. However, when a laminate was unified by these methods, the triboelectrically-charged filter was affected not a little by heat or moisture, and therefore, there was a tendency that the amount of electrical charge would become small, and the filtering efficiency would decrease. Further, in the process of conveying after the production of these charged filters, there was a tendency that when they were brought into contact with conveying rollers, the charge escaped from the charged filters through the conveying rollers, and the filtering efficiency decreased.
Furthermore, when the charged filters disclosed Patent literatures 1-3 were used for applications requiring flame-resistance, like the case where they were used as interior filters for automobile or the like, the flame-resistance was insufficient, and they could not be used for the applications.